PHILCO PHORUM

Notes on Model 116

Reports have been received by me of a circuit variation in some Model 116 receivers. The discrepancy is in the second detector circuit of the receiver, and also includes the volume control circuit.

No changes of this type were ever documented for Philco Service Bulletins 222 and 222A, Models 116B and 116X. The changes were also not documented in Philco's "Changes In Models" reports issued at the time. Now, I am able to document the changes for you.

While looking through the vast amount of Philco reference material I have here, I discovered a schematic of the Model 116PX radio-phonograph. This schematic HAS the changed circuitry shown on it. I have posted the complete schematic of the 116PX along with the rest of the Online Schematics.

Let us examine the changes in detail. As shown in the partial schematic below, earlier production of Model 116 used a simple second detector circuit, with a Type 37 connected as a diode by using the grid as a diode plate and the plate connected to the cathode.

Click image to enlarge

The 37 tube performs the dual function of detecting the audio signal and acting as the AVC diode. The AVC voltage, which appears as a negative voltage across resistor (70), is fed through a 2 megohm resistor (45) to the control grids of the set's RF and 1st IF tubes.

The volume control circuit is a bit unusual. The audio signal is controlled by the wiper arm of potentiometer (68) and sent to the grid of the 1st audio tube, a Type 77, through resistor (67), 500K, and capacitor (65), 0.05 uF. Normally, the audio signal would be coupled from the wiper arm of the potentiometer (volume control) through a coupling capacitor - no resistor - to the grid of the audio amplifier tube.

It should also be mentioned that the suppressor grids of the RF and 1st IF tubes are both grounded in the early version.

Now let us take a look at the later version's circuitry.

Click image to enlarge

Simple things first: In the later version, the audio is coupled from the volume control to the grid of the 1st audio tube in a conventional manner, through a 0.01 uF capacitor (65).

Notice the 2nd detector tube. There is a 110 pF capacitor connected between the plate and grid of this tube, not present in the early version. In this manner, the Type 37 is used as a double diode with plate and grid both used as diode plates.

The grid-cathode combination of the Type 37 detects the audio signal, while the plate-cathode combination acts as a separate AVC diode.

AVC voltage runs from the plate of the Type 37 to where it splits at resistors (45) and (81A), both 1 megohm. As shown above, resistor (45) carries the AVC line to the control grids of the RF and 1st IF tubes, as in the early version.

Now another major difference in the later version: Resistor (81A) carries an AVC signal to the suppressor grids of the RF and 1st IF tubes. The suppressor grids of these two tubes are grounded in the early version.

Final notes: The 116PX schematic, dated November 1, 1935, shows a 2000 ohm resistor (80) connected in parallel with the set's shadowmeter (78), which is also shown in the early version's schematic. Philco issued a production change on November 1, 1935, in which resistor (80) was removed from the circuit. Therefore, later versions with the different 2nd detector/volume control circuit should not have a resistor in parallel with the shadowmeter.

Hopefully this article, along with the 116PX schematic available from the Online Schematics page, will be of assistance to you when you need to service a later version of Model 116.